Damage to the neocortex during infancy often produces less of a behavioral deficit than similar damage sustained during adulthood. Some investigators have suggested that a similar compensation may result after early damage to the amygdala, and have reported that monkeys subjected to amygdalectomy during infancy may be indistinguishable from unoperated monkeys. Because previous studies did not examine many of the functions known to be disrupted after adult surgery, a research program was initiated seven years ago to determine whether all of these functions were equally compensated. Six infant Rhesus monkeys were subjected to bilateral amygdalectomy, and were compared in various individual, social, and learning situations with six unoperated monkeys o the same age. These tests demonstrated that monkeys operated during infancy had many of the deficits characteristic of operated adults, including hyperactivity, social submissiveness, and an active avoidance deficiency. In order to determine whether the deficits seen after early amygdalectomy were quantitatively, as well as qualitatively, similar to those seen after adult surgery, the normal control monkeys were subjected to bilateral amygdalectomy when they were six years old. Unpublished data collected thus far have demonstrated that there are no differences between early and late operated monkeys in amount of hyperactivity, or in degree of social submissiveness. The objectives of the proposed research program are (1) to compare the ability of early and late operated monkeys on active avoidance, and (2) to perform a histological analysis of the brain lesions, which will enable publication not only of the active avoidance data, but also of the data already collected on hyperactivity and social submissiveness.